The invention relates to using scanning interferometry to measure thickness(es), surface topography, and/or other characteristics of objects having complex surface structures, such as thin film(s).
Interferometric techniques are commonly used to measure the profile of a surface of an object. To do so, an interferometer combines a measurement wavefront reflected from the surface of interest with a reference wavefront reflected from a reference surface to produce an interferogram. Fringes in the interferogram are indicative of spatial variations between the surface of interest and the reference surface.
A scanning interferometer scans the optical path length difference (OPD) between the reference and measurement legs of the interferometer over a range comparable to, or larger than, the coherence length of the interfering wavefronts, to produce a scanning interferometry signal for each camera pixel used to measure the interferogram. A limited coherence length can be produced, for example, by using a white-light source, which is referred to as scanning white light interferometry (SWLI). A typical scanning white light interferometry (SWLI) signal is a few fringes localized near the zero optical path difference (OPD) position. The signal is typically characterized by a sinusoidal carrier modulation (the “fringes”) with bell-shaped fringe-contrast envelope. The conventional idea underlying SWLI metrology is to make use of the localization of the fringes to measure surface profiles. Scanning interferometers that use a limited coherence length to localize interference fringes in the interferometry signal are also referred to as “low coherence scanning interferometers.”
Typically, there are two approaches to processing such data. The first approach is to locate the peak or center of the envelope, assuming that this position corresponds to the zero optical path difference (OPD) of a two-beam interferometer for which one beam reflects from the object surface. The second approach is to transform the signal into the frequency domain and calculate the rate of change of phase with wavelength, assuming that an essentially linear slope is directly proportional to object position. See, for example, U.S. Pat. No. 5,398,113 to Peter de Groot. This latter approach is referred to as Frequency Domain Analysis (FDA).
If a low coherence scanning interferometer is used to collect a scanning interferometry signal from a sample having a thin film (e.g., a simple single-layer partially reflective film over an opaque substrate), and if the film is sufficiently thick, then the scanning interferometry signal will include two distinct regions of fringes corresponding to the upper and lower interfaces of the film. This is shown in FIG. 1, extracted from a reference by S. Petitgrand et al. (S. Petitgrand, A. Bosseboeuf, J. P. Gilles, P. Coste, P. Nérin, P. Vabre “Mesures 3D de topographies et de vibrations à l'échelle (sub)micrométrique par microscopie optique interférométrique” Proc. Club CMOI, Méthodes et Techniques Optiques pour l'Industrie (2002). A nearly identical paper can be downloaded from Fogale Nanotech website (http://www.fogale.com/acrobat/IEFCMOI2002_FR.pdf)). According to another paper by Bosseboeuf and Petigrand (Proc. SPIE 5145, 1-16, (2003)), the distance between these two signals is “Δ=n1d,” where here Δ is the distance between the maxima of the two regions of fringe contrast, d is the physical film thickness and n1 is the index of refraction.
Because the light passes through the film before reaching the substrate, there is a distortion in the apparent film thickness related to the refractive properties of the film. In prior-art references such as Bosseboeuf and Petigrand, the correction for this effect is to divide the apparent thickness by the index of refraction, to recover the true physical thickness of the film. Unfortunately, we often observe that this correction is insufficient.
In other applications, one is interested in the topology of the top and/or bottom surface of the film, instead or, or in addition to, the thickness of the thin film. Unfortunately, conventional processing of the low coherence scanning interferometry data can sometimes be corrupted by the presence of one or more underlying layers.